The present invention relates to a drill bit for use in rotary hammer drilling machines to make holes in rock, masonry and concrete. Such bits are well known and typically employ an axially extending shank having a pair of dust-removing helical grooves therein and hard metal cutting edges positioned at the forward end of the shank. The cutting edges generally extend over the entire diameter of the drill head in a variety of configurations and exert a grinding type action on the stone, masonry or concrete to cut into the material and form a borehole. In an effort to enhance the durability and precision of the drilling operation and reduce the wear on the cutting edges and jamming of the drill bit, different drill head cross sections have been employed and a wide variety of cutting edge configurations have been developed. Such edges are formed by a plurality of hard metal plates and/or pins which are embedded in the face of the shank, extend across the face and project axially therefrom. These cutting plates are frequently arranged in a variety of cross-shaped forms wherein one of the plates extends across the diameter of the shank to define the main cutting edges. This plate is often V or roof-shaped to define a centering point. The auxiliary cutting edges are formed from smaller plates projecting perpendicularly or at acute angles from the main plate. Examples of such drills are shown and discussed in U.S. Pat. Nos. 5,836,410; 5,482,124 and 3,960,223. While capable of forming boreholes in rock, masonry and concrete, these drill bits have certain shortcomings. Centering of the borehole becomes difficult as drilling progresses due to the overhang of the main cutting plate relative to the cylindrical sides of the drill head. As a result, it is difficult to maintain a straight and cylindrical borehole. Reinforcing steel (xe2x80x9crebarxe2x80x9d) also presents a problem as the roof-style points on the main cutting edge tend to slide off the rebar and allow the drills to progress against the sides of the reinforcing steel until the drill jams or breaks. The drill of the present invention overcomes the shortcomings in the prior art bits and thereby provides a new and improved hammer bit.
Briefly, the present invention comprises a drill bit for use in rotary hammer drilling machines which includes an axially extending shank having a plurality of discharge grooves therein, a centrally disposed cylindrical projection embedded in a convex end face of the shank, and terminating at its extended end in a pyramid-shaped point, and a plurality of primary cutters disposed about and spaced radially from the cylindrical projection. The pointed cylindrical projection permits precise location of the drill hole and minimizes wandering of the bit in creating a pilot hole. It also provides a chiseling effect to pulverize the rock, masonry or concrete as opposed to the grinding-type action of roof-shaped bits. The inclined surface of the pyramid point is multi-faceted and the facets are oriented so as not only to define the pyramid point but also to channel the comminuted material down the straight cylindrical side wall of the central projection and into the gaps between the central projection and the outer primary cutters where the material is compressed against the rock or other material being drilled and itself provides a grinding effect on the material being drilled to assist the primary cutters and pointed cylindrical projection in the drilling process and thus enhances the drilling speed. The configuration of the pyramid-shaped tip on the central projection also causes the drill to flare rebar upon contact and sit in the contact point so as to maintain a true drilling line and alert the driller to the presence of the rebar to prevent damage to the drill bit.
Accordingly, it is the object of the present invention to provide a drill bit for use in rotary hammer drilling machines which exhibits improved accuracy and enhanced drilling speed and which is less likely to be damaged when encountering rebar.
These and other objects and advantages of the present invention will become readily apparent from the following detailed description taken in conjunction with the accompanying drawings.